Congrats on the Synapse Fem 5! I am currently trying to hunt down a Synapse Carbon Fem 3, with no luck. I will likely end up waiting for the 2011's to come out.

I would think a little bit of soreness would be normal, especially if it's been 25 years since you've been on a road bike. Give your body a bit more time to adapt to the new geometry. For what it's worth...I too, have a chronic back condition (vertebral slippage at my L5/S1) so finding a comfortable bike that I can do long distances on is extremely important to me.

Happy riding!